1. Field of the Invention
The present invention relates to a device for measuring at least one parameter of a flowing fluid medium, in particular a fluid medium flowing through a flow pipe.
2. Description of Related Art
In many processes such as, for instance, in the field of industrial processing engineering, chemistry or machine construction, fluid media, especially masses of gas (e.g. an air mass) having particular properties (such as temperature, pressure, flow speed, mass flow, volume flow, etc.) have to be supplied. Among these are combustion processes, in particular, which run under regulated conditions.
An important application example is the combustion of fuel in internal combustion engines of motor vehicles, especially ones having subsequent catalytic exhaust gas purification, in which a certain air mass has to be supplied per unit time in a controlled manner (air mass flow). Various types of sensors are used to measure the air-mass throughput, in this instance. One sensor type known from the related art is what is generally known as a hot-film air mass sensor (HFM), which is described in one specific embodiment described in published German patent document DE 196 01 791 A1, for example. A sensor chip, which has a thin sensor diaphragm, e.g., a silicon sensor chip, is generally utilized in such hot-film air mass meters. At least one thermal resistor, which is surrounded by two or more temperature measuring resistors (temperature sensors), is typically situated on the sensor diaphragm. In an air flow that is routed across the diaphragm there is a change in the temperature distribution, which in turn is detectable by the temperature measuring resistors and is able to be evaluated with the aid of a control and evaluation circuit. Thus, for instance, an air mass flow is able to be determined from a difference in resistance of the temperature measuring resistors. A number of other variations of this sensor type is known from the related art. The present invention is also not limited to the sensor type of the hot film air mass sensor described, but may basically be used as most types of sensors being utilized as stationary installed sensors or as plug-in sensor in a flowing medium.
However, in the plug-in sensor constructions described in the related art it is disadvantageous that the plug-in sensors described, often cause problems in the intake tract with regard to a pressure loss attributable to flow resistance. This means, in particular, that the reproducibility of the signals of such sensors is not optimal.
Many sensors, particularly hot film air mass sensors, are supplied, in practice, with a grating or a grating combination. These gratings may be integrated into a flow pipe, for example, and are usually positioned a few centimeters upstream of the plug-in sensor or sensor in the flow, and they have the task to even out the speed profile in the flow pipe. Such gratings furthermore have the task of removing any swirl that may possibly be present from the flow. The evening-out effect of the grating is achieved by its braking effect on the flow. A small-scale turbulence is produced at the same time, which mixes fast and slow flowing fluid, and thus contributes to a speed equalization over the entire pipe cross section. The result is that the characteristics curve of the sensor (for instance, a relationship between air mass and output frequency or output voltage) is nearly independent of the speed profile of the inflowing air.
One example of such a device having a grating is known from published German patent document DE 196 47 081 A1. In contrast to known gratings, in which equal-sized, equidistant flow openings are provided, it is proposed, for these gratings, that flow openings having different flow-through cross sections be provided. The flow-through cross sections are adjusted to the inflow, so as to effect a flow downstream from the grating having an essentially even speed distribution.
Unfortunately, however, many sensors, particularly many air mass sensors, in combination with known grating constructions, demonstrate characteristics curves in some air mass ranges whose reproducibility leaves room for further improvement. The cause for this is usually an instability of the flow directly after the grating crossties. In the wake of the grating crossties, detachment areas form and irregularities in the speed profile, which are unstable in principle, so that the measured values of the sensors situated downstream from the grating constructions may also be unstable.